Transimpedance amplifier

ABSTRACT

A method of forming a circuit includes forming a transimpedance amplifier having a first input node and a second input node. The method also includes forming a feedback circuit having a first transistor having a drain terminal connected to the first input node, a source terminal, and a gate terminal, a second transistor having a drain terminal connected to the second input node, a source terminal, and a gate terminal, and a third transistor having a drain terminal connected to the source terminal of the first transistor and the source terminal of the second terminal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of application Ser. No. 13/226,650, filed Sep. 7, 2011, incorporated herein by reference in its entirety.

FIELD OF INVENTION

The present invention relates generally to transimpedance amplifiers.

DESCRIPTION OF RELATED ART

Transimpedance amplifiers (TIAs) are often used to convert an input current signal and into a corresponding output voltage signal. Typical TIAs are often used in systems that receive a current signal from a sensor device. The output current signal from the sensor device is received by the TIA and converted into a corresponding voltage signal that may be processed by a processor. For example, optical receivers often include a photodiode device that outputs a current signal in response to being exposed to a source of radiation such as light. The TIA receives the current signal output from the photodiode, converts the current signal into a corresponding voltage signal, and outputs the voltage signal. The voltage signal may be amplified by an amplifier, such as a linear amplifier that outputs the amplified voltage signal to a clock and data recovery block. The clock and data recovery block converts the amplified voltage into a digital data signal and recovers an associated clock signal from the data rate of the digital data. The data signal and the clock signal may be output to a processor device or logic circuit.

BRIEF SUMMARY

According to one embodiment a method of forming a circuit includes forming a transimpedance amplifier having a first input node and a second input node. The method also includes forming a feedback circuit having a first transistor having a drain terminal connected to the first input node, a source terminal, and a gate terminal, a second transistor having a drain terminal connected to the second input node, a source terminal, and a gate terminal, and a third transistor having a drain terminal connected to the source terminal of the first transistor and the source terminal of the second terminal.

According to another embodiment a method of forming a circuit includes forming a transimpedance amplifier having a first input node, a second input node, a first output node, and a second output node. The method includes forming a feedback circuit comprising a first transistor, a second transistor, and a third transistor. The method also includes connecting a drain terminal of the first transistor to the first input node, connecting a drain terminal of the second transistor to the second input node, and connecting a drain terminal of the third transistor to a source terminal of the first transistor and a source terminal of the second transistor. The method further includes forming an amplifier having a first input node connected to the first output node of the transimpedance amplifier, a second input node connected to the second output node of the transimpedance amplifier, and an output node.

According to yet another embodiment a method of using a transimpedance amplifier includes receiving an input current signal from a current output device by the transimpedance amplifier. The method also includes drawing a feedback current signal from the current output device by a feedback circuit, outputting a voltage output signal from the transimpedance amplifier based on the input current signal, and filtering the voltage output signal to create a voltage feedback signal which is input into the feedback circuit. The output of the current output device consists of the input current signal and the feedback current signal and the feedback circuit is configured to determine a magnitude of the feedback current signal based on the voltage feedback signal.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a prior art example of a circuit.

FIG. 2 illustrates a block diagram of an exemplary embodiment of an optical receiver system.

FIG. 3 illustrates a block diagram of an exemplary embodiment of a portion of the system of FIG. 2.

FIG. 4 illustrates a circuit diagram of an exemplary embodiment of a TIA and differential circuit.

FIG. 5 illustrates a graph of the V_(FB) to I_(FB) characteristic of the circuit illustrated in FIG. 4.

FIG. 6 illustrates an exemplary embodiment of a single-ended TIA with a CMOS inverter and a CMOS inverter transconductor circuit.

FIG. 7 illustrates a graph of the V_(FB) to I_(FB) characteristic of the illustrated embodiment in FIG. 6.

FIG. 8 illustrates a flowchart diagram of a method for forming a circuit in accordance with an exemplary embodiment.

FIG. 9 illustrates a flowchart diagram of a method of using a transimpedance amplifier in accordance with an exemplary embodiment.

DETAILED DESCRIPTION

To reliably convert an input current into a digital output, the average, or DC input current is determined and used as a threshold to differentiate input currents below the average (digital 0s) from input currents above the average (digital 1s). Removing the DC input current is known as level restoration, as it restores the DC level to zero to provide the threshold. Any DC input offset voltage that exists in a transimpedance amplifier (TIA) or amplifier such as, for example, a limiting amplifier, should also be compensated since the input offset voltage effectively shifts the threshold between 0 and 1. A preferred threshold has the greatest possible distance between the 0 and 1 levels, which allows the greatest space for signal noise or other non-idealities to corrupt the signal without resulting in errors in the received data. The space between the threshold and the 0 and 1 levels is called the noise margin. If the threshold is not optimally set, the noise margin may be degraded and the probability of an error increases (i.e. the bit error rate (BER) increases).

A feedback loop is often used to provide level restoration and DC offset compensation in a circuit with a TIA and limiting amplifier (LA). In this regard, the feedback loop measures the DC level at the output of the LA and returns a proportional signal to the input of the TIA, which forces the output DC level to either a reference voltage (for a single ended output) or to a differential zero (for a differential output). The output DC level is forced to an ideal threshold, thereby level restoring the input DC current and compensating for the TIA and LA DC input offsets, maximizing the noise margin, and minimizing the BER.

FIG. 1 illustrates a prior art example of a circuit 100 with a TIA 104 and LA 106 circuit with a low pass filter (LPF) arrangement 102. The feedback loop may be implemented using low pass filter 102 such as, for example, a resistor capacitor (RC). The LPF 102 extracts the DC content of the output and returns the DC content to the TIA 104 to be subtracted from the input DC level and the TIA and LA DC offset. The TIA 104 has two inputs, with one input connected to the input device 108 and the second input connected to the feedback loop. The subtraction of the DC content occurs within the TIA 104.

In prior art examples of complementary metal oxide semiconductor (CMOS) TIA circuits, an additional NMOS transistor (not shown) may be used to subtract the DC input current and any DC offset in the TIA. When such a circuit is used in an analog feedback system, the circuit performs poorly because the feedback loop gain depends on the bias point of the NMOS transistor. The varying feedback loop gain causes the frequency response of the feedback loop to change as the DC input current and the TIA and LA offsets vary, which negatively affects the circuit performance in an analog feedback system. The circuit arrangement may only subtract current from the input node, thus providing level restoration only when the input DC current is positive (flowing into the input node). When the DC current is negative (flowing out of the input node) the circuit cannot provide level restoration.

FIG. 2 illustrates a block diagram of an exemplary embodiment of an optical receiver system 200. The system 200 includes a TIA 202, an amplifier 204, that may include an amplifier such as, for example, a limiting amplifier, a clock and data recovery block (CDR) 206, a current output device 208 that may include any type of device that outputs a current such as, for example, a photo diode, and a feedback circuit 210 that receives a feedback voltage (V_(FB)) from a low pass filter (LPF) 205. The LPF 205 may be connected to either the output of the amplifier 204 or the output of the TIA 202. The features of the system 200 may be arranged as separate components or with any combination of features arranged as integrated components in an integrated circuit chip. In the illustrated embodiment, the system 200 outputs signals to a processor device or logic circuit 212 that may also be included on in integrated circuit or on a separate circuit or device.

In operation, the current output device 208 outputs a current signal (I_(IN)) to the TIA 202 and the feedback circuit 210. The TIA 202 converts I_(IN) into corresponding voltage signal (V_(out))that is output to the amplifier 204. The amplifier 204 amplifies the V_(out) signal and outputs an amplified signal V_(outAMP) to the CDR 206. The V_(outAMP) signal (or in alternate embodiments, the V_(out) signal from the TIA 202) DC content is measured by the LPF 205 and the DC content (signal V_(FB)) is sent to the feedback circuit 210. The CDR 206 converts the amplified voltage into a digital data signal and recovers an associated clock signal from the data rate of the digital data. The CDR outputs a data signal V_(data) and a clock signal V_(clock) to the processor 212.

FIG. 3 illustrates a block diagram of an exemplary embodiment of a portion of the system 200. In this regard, the system 200 is implemented as a circuit having I_(IN−) and I_(IN+) inputs. In the illustrated embodiment, the I_(IN+) input may be connected to the current output device 208 (of FIG. 2) and the I_(IN−) input may be floating. The TIA 202 outputs signals V_(OUT+) and V_(OUT−) where the V_(OUT+) signal may be connected to the amplifier 204 (of FIG. 2) and the V_(OUT−) signal may be floating, alternatively both V_(OUT+) and V_(OUT−) may be connected to the amplifier 204 as a differential signal V_(OUT). The feedback circuit 210 in the illustrated embodiment includes a first transistor (T_(D1)) 302 that is connected to the I_(IN−) node 301 and a second transistor (T_(D2)) 304 that is connected to the I_(IN+) node 303. A third transistor (T_(T)) 306 is biased by a bias voltage (V_(BIAS)) (e.g., a bias voltage source such as a constant or relatively constant voltage) and is connected to the T_(D1) 302 and the T_(D2) 304 at a node 305. Transistors T_(D1) 302, T_(D2) 304, and T_(T) 306 may include any type of transistor, for example, including but not limited to, bipolar transistors and field effect transistors. For the embodiments described herein, though the figures and descriptions include field effect transistors, one of ordinary skill in the art would understand that any of the field effect transistors may be replaced with a variety of other types of transistors, such as, for example, bipolar transistors in alternate embodiments. In this regard, the source, drain, and gate terminals of the field effect transistors described herein correspond to emitter, collector, and base terminals respectively for embodiments that include bipolar transistors and may be used interchangeably when describing the embodiments. The use of the terms source, drain, and gate terminals is thus not limiting to embodiments that include only field effect transistors and may include terminals for any similar corresponding transistor terminals (e.g., emitter, collector, and base terminals respectfully) for other types of transistors understood by one of ordinary skill in the art. In operation, the V_(BIAS) applied to the T_(T) 306 provides a constant current I_(TAIL). The I_(TAIL) is steered by the input voltages V_(FB+) and V_(FB−) through the transistors T_(D1) 302 and T_(D2) 304 to produce currents I_(FB+) and I_(FB−). The currents I_(FB+) and I_(FB−) subtract from the currents I_(IN+) and I_(IN−) at the nodes 303 and 301 respectively, to remove the DC input current and to provide level restoration. The arrangement may also be used to compensate for DC offsets in the TIA 202 and the amplifier 204.

FIG. 4 illustrates a circuit diagram of an exemplary embodiment of a TIA 402 and feedback circuit 410 that correspond to the TIA 202 and feedback circuit 210 of the system 200 (of FIG. 2). In this regard, the TIA 402 is a CMOS inverter TIA that include a first single-ended inverter portion 414 and a second single-ended inverter portion 416. The first single-ended inverter portion 414 includes transistors M_(P1) 418 and M_(N1) 420, and a resistor R_(F1) 422. The second single-ended inverter portion 416 includes transistors M_(P2) 424 and M_(N2) 426, and a resistor R_(F2) 428.

The feedback circuit 410 includes differential pair transistors M_(D1) 430 and M_(D2) 432 and a tail transistor M_(T) 434. The V_(OUT−) node 401 is connected to the resistor R_(F1) 422 and the drain terminals of the M_(P1) 418 and M_(N1) 420. The I_(IN+) node 403 is connected to the gate terminals of the M_(P1) 418 and M_(N1) 420, the R_(F1), and the drain terminal of the M_(D1) 430. The V_(OUT+) node 405 is connected to the resistor R_(F2) 228 and the drain terminals of the M_(P2) 424 and M_(N2) 426. The I_(IN−) node 407 is connected to the gate terminals of the M_(P2) 424 and M_(N2) 426, the R_(F2) 228 and the drain terminal of the M_(D2) 432. The node 409 is connected to the drain terminal of the M_(T) 434 and the source terminals of the M_(D1) 430 and M_(D2) 432. The M_(T) 434 receives a bias voltage V_(BIAS) the gate terminal. The node 401 (V_(OUT−)) outputs V_(OUT−) and may be connected to an amplifier, such as, for example, amplifier 202 (of FIG. 2). A low pass filter LPF, such as, for example the LPF 205 (of FIG. 2) may receive V_(OUT−) prior to, or following amplification (e.g., V_(outAMP) output by the amplifier 204) and is connected to the gate terminal of the MD1 430, which provides V_(FB−) to the M_(D1) 430. The node 405 (V_(OUT+)) outputs V_(OUT+) and may be connected to an amplifier, such as, for example, amplifier 202 (of FIG. 2). A low pass filter LPF, such as, for example the LPF 205 (of FIG. 2) may receive V_(OUT+) prior to, or following amplification (e.g., V_(outAMP)) and is connected to the gate terminal of the M_(D2) 432, which provides V_(FB+) to the M_(D2) 432. The operation of the illustrated embodiment is similar to the operation described above in FIG. 3.

FIG. 5 illustrates a graph 500 of the V_(FB) to I_(FB) characteristic of the circuit illustrated in FIG. 4. In this regard, where V_(FB)=(V_(FB+)−V_(FB−)) and I_(FB)=(V_(FB+)−V_(FB−)) there exists a linear region 501 where the current may be divided between the two sides of the feedback circuit 410 with constant transconductance. The constant transconductance provides a constant gain for the feedback input, which keeps the feedback loop response stable as I_(FB) and V_(FB) vary within the linear region 501. The gain and the linear range of the feedback circuit 410 may be adjusted by changing the V_(BIAS) voltage, which allows compensation for process and environmental variations. The differential current may be positive or negative allowing the feedback circuit 410 to compensate for a negative single-ended DC input current by subtracting more current from the opposite polarity input, thus balancing the subtracted current from both halves of the TIA 402 to output a balanced differential signal.

FIG. 6 illustrates an exemplary embodiment of a single-ended TIA 602 with a CMOS inverter and a feedback circuit 610. In this regard, the TIA 602 portion includes transistors M_(P) 604 and M_(N) 606 and a resistor R_(F) 608. The feedback circuit 610 includes transistors M_(FP) 612 and M_(FN) 614. In the illustrated embodiment, the node 603 is connected to the I_(IN), the R_(F) 608; the gate terminals of the M_(P) 604 and M_(N) 606; and the drain terminals of the M_(FP) 612 and M_(FN) 614. The node 601 is connected to the V_(OUT); the drain terminals of the M_(P) 604 and M_(N) 606 and the R_(F) 608. The node 605 is connected to the gate terminals of the M_(FP) 612 and M_(FN) 614 and a low pass filter LPF, such as, for example the LPF 205 (of FIG. 2), which may receive V_(OUT) prior to, or following amplification (e.g., V_(outAMP) output from the amplifier 204). The single-ended TIA 602 corresponds to the TIA 202 block (of FIG. 2) while the feedback circuit 610 corresponds to the feedback circuit block 210 and may be incorporated into an alternate embodiment of the system 200 described above.

In operation, the feedback circuit 610 provides feedback input for DC offset cancelation and restoration. In this regard, the input V_(FB) adjusts the current I_(FB) flowing from the node 603. The current I_(FB) subtracts from the current I_(IN) at the node 603 allowing removal of the DC input current and providing level restoration. FIG. 7 illustrates a graph 700 of the I_(FB) to V_(FB) of the illustrated embodiment in FIG. 6. The linear region 701 illustrates the linear region where current I_(FB) may take on positive or negative values with constant transconductance. The constant transconductance provides a constant gain for the feedback input, which keeps the feedback loop response stable as I_(FB) and V_(FB) vary within the linear region 701.

Referring now to FIG. 8, a flowchart diagram of a method 800 for forming a circuit in accordance with an exemplary embodiment is shown. As shown at block 802, the method 800 includes forming a transimpedance amplifier having a first input node and a second input node. Next, as shown at block 804, the method 800 includes forming a feedback circuit comprising a first transistor, a second transistor, and a third transistor. As shown at block 806, the method 800 includes connecting a drain terminal of the first transistor to the first input node. Next, as shown at block 808, the method 800 includes connecting a drain terminal of the second transistor to the second input node. As shown at block 810, the method 800 includes connecting a drain terminal of the third transistor to a source terminal of the first transistor and to a source terminal of the second transistor. Next, as shown at block 812, the method 800 includes connecting the first output node of the transimpedance amplifier to a gate terminal of the first transistor via a low pass filter. As shown at block 814, the method 800 includes connecting the second output node of the transimpedance amplifier to a gate terminal of the second transistor via the low pass filter.

Referring now to FIG. 9, a flowchart diagram of a method 900 of using a transimpedance amplifier in accordance with an exemplary embodiment is shown. As shown at block 902, the method 900 includes receiving an input current signal from a current output device by the transimpedance amplifier. Next, as shown at block 904, the method 900 includes drawing a feedback current signal from the current output device by a feedback circuit. As shown at block 906, the method 900 includes outputting a voltage output signal from the transimpedance amplifier based on the input current signal. Next, as shown at block 908, the method 900 includes filtering the voltage output signal to create a voltage feedback signal which is input into the feedback circuit. As shown at block 910, the method 900 the output of the current output device consists of the input current signal and the feedback current signal and the feedback circuit is configured to determine a magnitude of the feedback current signal based on the voltage feedback signal.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 

What is claimed is:
 1. A method of forming a circuit comprising: forming a transimpedance amplifier having a first input node and a second input node; forming a feedback circuit comprising a first transistor, a second transistor, and a third transistor; connecting a drain terminal of the first transistor to the first input node; connecting a drain terminal of the second transistor to the second input node; and connecting a drain terminal of the third transistor to a source terminal of the first transistor and to a source terminal of the second transistor.
 2. The method of claim 1, wherein the third transistor comprises a gate terminal connected to a bias voltage source.
 3. The method of claim 1, wherein the transimpedance amplifier further comprises a first output node and a second output node.
 4. The method of claim 3, further comprising connecting the first output node to a gate terminal of the first transistor via a low pass filter.
 5. The method of claim 4, further comprising connecting the second output node to a gate terminal of the second transistor via the low pass filter.
 6. The method of claim 1, further comprising forming a current output device connected to the first input node.
 7. The method of claim 6, wherein the current output device is a photodiode.
 8. The method of claim 4, wherein the transimpedance amplifier further comprises a first single-ended inverter portion and a second single-ended inverter portion.
 9. The method of claim 3, wherein the transimpedance amplifier further comprises a first single-ended inverter portion that comprises: a fourth transistor having a gate terminal connected to the first input node and a drain terminal connected to the first output node; a fifth transistor having a gate terminal connected to the first input node and a drain terminal connected to the first output node; a first resistor connected to the first input node and the first output node.
 10. The method of claim 9, wherein the transimpedance amplifier further comprises a second single-ended inverter portion that comprises: a sixth transistor having a gate terminal connected to the second input node and a drain terminal connected to the second output node; a seventh transistor having a gate terminal connected to the second input node and a drain terminal connected to the second output node; a second resistor connected to the second input node and the second output node.
 11. A method of forming a circuit comprising: forming a transimpedance amplifier having a first input node, a second input node, a first output node, and a second output node; forming a feedback circuit comprising a first transistor, a second transistor, and a third transistor; connecting a drain terminal of the first transistor to the first input node; connecting a drain terminal of the second transistor to the second input node; connecting a drain terminal of the third transistor to a source terminal of the first transistor and a source terminal of the second transistor; and forming an amplifier having a first input node connected to the first output node of the transimpedance amplifier, a second input node connected to the second output node of the transimpedance amplifier, and an output node.
 12. The method of claim 11, further comprising forming a clock and data recovery circuit having an input node connected to the output node of the amplifier, a data signal output node and a clock signal output node.
 13. The method of claim 12, further comprising forming a processor portion having a first input node connected to the data signal output node and a second input node connected to the clock signal output node.
 14. The method of claim 11, further comprising forming a current output device connected to the first input node.
 15. The method of claim 14, wherein the current output device is a photodiode.
 16. A method of using a transimpedance amplifier comprising: receiving an input current signal from a current output device by the transimpedance amplifier; drawing a feedback current signal from the current output device by a feedback circuit; outputting a voltage output signal from the transimpedance amplifier based on the input current signal; and filtering the voltage output signal to create a voltage feedback signal which is input into the feedback circuit, wherein an output of the current output device consists of the input current signal and the feedback current signal, and wherein the feedback circuit is configured to determine a magnitude of the feedback current signal based on the voltage feedback signal.
 17. The method of claim 16, wherein the feedback circuit comprises: a first transistor having a drain terminal connected to a first input node of the transimpedance amplifier; a second transistor having a drain terminal connected to a second input node of the transimpedance amplifier; and a third transistor having a drain terminal connected to a source terminal of the first transistor and a source terminal of the second transistor.
 18. The method of claim 17, wherein the third transistor further comprises a gate terminal connected to a bias voltage source.
 19. The method of claim 16, wherein filtering the voltage output signal is performed by a low pass filter.
 20. The method of claim 16, wherein, the current output device is a photodiode. 